In Fourth Generation, 4G, Evolved Packet Core, EPC, systems, User Equipment, UE, may attach to the 4G/EPC system to gain connectivity towards Packet Data Networks, PDNs. A PDN connection is defined as the association between a UE, represented, for example, by an Internet Protocol, IP, address, and a Packet Data Network, PDN, represented by an Access Point Name, APN.
An APN is a label according to Domain Name System, DNS, naming conventions, describing the access point to the packet data network. The list of authorized/subscribed APNs for a given UE together with the characteristics of the PDN connections for each APN, such as IP vs non-IP type, default Quality of Service, QoS, etc., is stored in a Home Subscriber Server, HSS, of the telecommunication system as part of the subscription information for each UE or subscriber.
The subscription profile information may contain at least the configuration for a default APN. There may be at most two default APNs for a given UE, such as one default APN for PDNs of an “IF” type and another default APN that may belong to PDNs of a “Non-IP” type.
Up to 3rd Generation Partnership Project, 3GPP, Release 15, 3GPP Rel-15, for delivering a first set of 5G standards, during an Attach procedure, the UE shall include a PDN connectivity request procedure, and the EPC Core Network shall establish a PDN connection for the default APN when no APN is provided by the UE. A UE connected in Long Term Evolution, LTE, /EPC shall have at least one active PDN connection, otherwise the LTE/EPC will detach the UE from the system.
A Voice over LTE, VoLTE, UE, based on IR.92 IP Multimedia Subsystem, IMS, Profile for Voice and Short Message Service, SMS, supports the use of an APN during the Attach procedure and also to rely on the default APN for the first PDN connection that has to be established.
In 3GPP Rel-15, a feature allowing “Attach without PDN Connectivity” has been introduced, where a UE may be connected to the LTE/EPC system without any active PDN connection. This feature is controlled by an additional “PDN-Connection-Restricted” flag. This flag, when set, and if the Mobility Management Entity, MME, and the UE support Attach without PDN connection, indicates the MME that same shall not establish any non-emergency PDN connection for the user.
The possibility for a UE to attach to a 5G telecommunication network without an active PDN connection, has been made the baseline mode of operation in 5G, except for the fact that the concepts of PDN Connection and APN have been renamed to Packet Data Unit, PDU, Session and Data Network Name, DNN, respectively.
Furthermore, according to current versions of the technical specifications 3GPP TS 23.501 and 23.502, PDU sessions can only be established upon UE request. At most, upon request from an Application Server, AS, 5GC is able to trigger a specific application in the UE. The UE, when receiving such trigger message, shall pass same to the identified application in the UE, where the identified application in the UE may establish a PDU Session to a specific DNN.
Currently there is no mechanism defined in 3GPP for 5GC itself to trigger the UE to establish a PDU session in 5GC. In other words, it is currently not possible to have the same behavior with 5GC as with EPC in relation to the handling of default APN/DNN. In case the user subscription marks the IMS APN/DNN as default APN, 5GC will not trigger the establishment of the DNN during registration in 5GC.
The existing mechanisms for PDU session establishment are either UE initiated or network, NW, initiated after invocation of an Application Server external to the 5GC network, and even in this NW initiated method the actual PDU session is established by the UE.
The currently defined network initiated PDU session establishment procedure by 3GPP is suitable as a device trigger mechanism for Internet of Things, IoT, devices and for which this mechanism requests the UE to establish PDU sessions triggered by an IoT application platform. This device trigger request is completed by sending an SMS to the UE including the information required for the UE, i.e. the 5GC, to support SMS.
However, for certain services, such as but not limited to, IP Multimedia Subsystem, IMS, voice services, the network initiated PDU session establishment procedure presently defined in 3GPP is not suitable. These devices are expected to be connected running the PDU session towards the IMS at all times. In the absence of support for a network initiated PDU session establishment procedure in 5GC, the PDU session establishment process is delegated to the UE. An appropriate UE configuration is therefore advantageous for proper service delivery and user experience.